fitting multiple data sets with variable norm

I have three spectra from XMM MOS1, MOS2, and MOS3.  I want to fit a 
Galactic-absorbed powerlaw to all three simultaneously, but I want to 
allow the normalization to differ for each spectrum due to cal issues.

I want to evaluate confidence ranges for the Gamma parameter for this fit.

I saw the description in the "fit_counts()" help text, but I'm not sure 
it applies directly.

Also, I'm using an old version (1.4.9-38), so it's good if there's some 
tried and true way of doing this:

Thanks!
Rob
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